This invention relates to flexible polyurea foams. More specifically, it concerns controlling the load bearing properties of such flexible foams.
Flexible polyurea foams are used in a wide variety of applications, particularly as cushioning such as in automobile seats, headrests, dashboards and the like, in furniture, bedding, carpet padding, packaging, toys and the like. The ability to control load bearing properties of such foams is important because each application presents its own requirements of support and/or perceived softness. For instance, an automobile seat should support a person sufficiently to cushion him from road bumps, and should also be comfortable.
Foam-forming formulations are carefully chosen to produce predetermined load bearing properties. Load bearing is, for instance, generally increased by adding fillers, using copolymer polyols, using chain extenders, increasing crosslinking, increasing density or increasing the isocyanate index. These methods of increasing load bearing strength, however, often increase the cost of a foam by requiring use of additional materials. Physical properties or handling properties are also often affected. For instance, viscosity of a foam-forming formulation may often be increased by addition of fillers, copolymer polyols and the like.
Polyurea foams are generally advantageous when relatively high load bearing strengths are needed. The advantages of polyurea formulations over polyurethane formulations that produce similar load bearing characteristics frequently include lower isocyanate index, lower viscosity because copolymer polyols are often unnecessary, and less foam shrinkage since less crosslinker is generally required.
Furthermore, certain foam applications, such as comfort cushioning, frequently require that the foam used have different load bearing strengths in different parts of the foam. For instance, seating may be more comfortable when one sits directly on a relatively less load bearing (generally perceived as softer) section of a seat, but that section is surrounded by relatively harder cushioning. Foam having different load bearing strengths in different areas of the foam is often referred to as dual hardness foam, although more than two regions having different hardness may be present. Such foams may be formed by various methods including pouring foams of differing compositions into a common mold as described, for instance, by M. J. Brown in "The Industrial Production of Dual Hardness Foam Seats Using Polymer Polyols, Robots and Unmodified Moulds," in Polyurethanes World Congress 1987, pp. 538-543 (1987) and U.S. Pat. No. 4,714,574, which is incorporated herein by reference. The use of differing compositions, however, is often accompanied by physical separation of parts of a foam having different compositions.
It is desirable to overcome or ameliorate difficulties such as separation caused by differences in curing dual hardness foams. Furthermore, it is desirable to increase the load bearing strength of polyurea foam, especially to do so with less shrinkage, formulation viscosity or concentration of isocyanate than is frequently required when increased load bearing is desired.